The present invention relates to pumping systems for delivering fluid from vehicle-carried tanks, such as pumping systems for delivering liquid propane (or, more commonly and specifically, liquid petroleum gas (“LPG”), and the term “propane” as used herein includes LPG and similar fuels containing propane) from a bobtail truck. Liquefied compressed gases such as propane are commonly transported in a tank called a “bottle” carried on a truck known as a “bobtail”. Bottles typically have a capacity of about 2,600 to 3,000 gallons, though some bobtails can include a bottle up to about 5,000 gallons.
The bobtail delivery/distribution of propane is a significant portion of the product cost, and bobtail delivery of propane needs to be made as efficient as possible. Because both bobtail trucks and driver time are expensive, it is important to manage the bobtail fleet as efficiently as possible, and thereby deliver as much propane as quickly as possible, with as few trucks as possible. For best utilization of the bobtail fleet, the bobtail trucks must be extremely reliable. Maintenance of the fleet to effectively eliminate downtime is critical. As delivery schedules get tighter and tighter, delays or difficulty in delivering propane not only significantly reduces profitability of the fleet, but also angers customers. At present, bobtail drivers can average 24 stops each shift, to deliver up to two full bottles or more—averaging about 4,800 gallons—per shift, for a total average delivery of 450,000 or more gallons per bobtail per year. Some bobtails may deliver 1.3 million gallons or more in a single year.
A typical bobtail weighs around 33,000 pounds when fully loaded, and is carrying about 17,000 pounds due to the weight of the bottle after the propane has been delivered. The bobtail trucks need to be rugged and robust, withstanding the heavy loads. Over the last few decades, heavier duty, diesel powered, class 7 trucks have replaced lighter, gasoline powered engines.
One of the ways in which bobtail performance affects delivery times is in how long it takes to pump propane out of the bottle and into a customer's tank, often called a “pig”. Bobtail trucks typically include a product pump which is used to maintain pressure and move propane from the bobtail's bottle into the customer's pig. In some cases, the product pump is powered directly from a power take off or PTO on the engine/transmission of the bobtail, which involves a mechanical transfer (typically a rotating shaft) from the PTO to the product pump which is typically located on the front bottom of the bottle, with a delivery pipe extending from the product pump to a delivery hose on the rear of the vehicle. In other cases, the product pump is located on the back of the bobtail, and power is delivered from the PTO on the bobtail engine/transmission to the product pump using a hydraulic fluid. Hydraulic systems can be quieter than mechanically powering the product pump off the PTO, and have been found to be very reliable. Delivery speeds and pump times using hydraulic systems have been good, and pump life also appears to be good. Additionally, once the hydraulic system is in place, hydraulics can be used for more than just the product pump. For instance, product hose reels could be operated hydraulically, and the remote shutdown system can be incorporated into the hydraulics.
One way to improve the performance of the hydraulic power and product pump is to use the Hydraulic Oil Cooler Supplying Vessel Pressure Stabilizer system as disclosed in U.S. Pat. No. 6,732,791. The arrangement disclosed in U.S. Pat. No. 6,732,791 provides numerous benefits in pumping fluid from a vehicle-carried tank. This system includes a liquid-to-liquid heat exchanger which cools the hydraulic fluid using a small return (approximately 2 gpm) of product to the bottle. The heated propane return to the bottle increases product pump flow rates, reduces cavitation and increases product pump life. The preferred system also eliminates the need for a fan to cool the hydraulic oil. In this prior art arrangement, the ultimate power source driving the fluid is the engine of the vehicle. A PTO draws power from the vehicle engine to drive the hydraulic side of the arrangement. The hydraulic power is then used to drive the main discharge pump for the bottle. The present invention is particularly contemplated as an improvement to U.S. Pat. No. 6,732,791, assigned to the assignee of the present invention and incorporated by reference.
At the same time as reliability is paramount, longevity of each truck is also very important for profitability. A bottle can be used for 30 years or more—typically two to three times longer than the life of the bobtail itself. Thus bottles need to be able to be switched between bobtail trucks as efficiently and easily as possible. The bobtail trucks themselves might log 200,000 to 300,000 miles over an average 12 year lifespan. In the same way as saving a few cents per bobtail at each delivery location can add up to millions of dollars in savings for the entire fleet, adding a few years or few thousand miles to the lifespan of each bobtail can also add up to millions of dollars in savings for the entire fleet. More ways to save are needed.